To date, IPCC scientists and others have said increased photosynthesis due to higher carbon dioxide levels could offset crop losses brought about by higher temperatures and lower soil moisture in a warmer global climate.

But Leakey and colleagues predict the 'fertilising effect' of rising CO2 will be only half as much as has been thought by 2050.

"What we're saying is the fertilisation effect of CO2 will not be as good a thing as was previously thought," says Leakey. "The current predictions are over-optimistic."

The researchers say the IPCC's data come from old experiments on crops grown in greenhouses and shelters that don't properly capture real field conditions.

Studies using special field experiments show the actual benefit from rising CO2 to world staples, such as corn, rice, sorghum, soybeans and wheat, they say.

The field experiments involve Free-Air Concentration Enrichment technology experiments, in which crops are grown in a standard field, but are surrounded by special contraptions that allow CO2 to be released across them to simulate increasing atmospheric concentrations.

"It's very much more real world," says Leakey.

He says unlike in glasshouse experiments, the plants receive natural light, wind, temperature, rainfall and have as much root volume as they need.

The field experiments also allow for pests and diseases that are part of real world conditions.

"When you do these experiments in the field under realistic simulations of future conditions we see about 50% of the response [to increasing CO2] that was previously seen," says Leakey.

Leakey says the results are averaged from a number of field experiments involving different crops in the US, Switzerland, New Zealand and Japan.

He says good data is not available for the tropics and more research on this and on other areas, such as wheat production in Australia is needed.

There also needs to be more field experiments to study the impact of changing ozone, temperature and drought on agriculture, says Leakey.

He also says it may be possible to breed crops that are better able to make use of increasing CO2 levels.